Reciprocating compressor

ABSTRACT

In a reciprocating compressor with delivery rate control the actuation device ( 3 ) for the valve unloader ( 2 ) of the suction valve ( 1 ) to be held open comprises as drive an electric motor ( 5 ), whose driven element ( 6 ) in conjunction with the non-rotating lifting element ( 10 ) of the valve unloader ( 2 ) and the rolling elements ( 8 ) arranged in between forma ball screw drive. This results in a small-dimensioned, precisely to be regulated actuation device ( 3 ) with high actuating forces and with high motion dynamic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a reciprocating compressor with capacity control control, comprising a valve unloader arranged at at least one automatically acting suction valve of the compressor, said valve unloader holding open at least one sealing element of the suction valve over a portion of the compressor's cycle by means of an electrically operated actuation device, as well as a method for controlling the capacity of such a reciprocating compressor.

2. The Prior Art

To control the capacity of reciprocating compressors, preferably running at a constant rotational speed, one frequently resorts to the so-called reverse flow control, in which at least one suction valve per cylinder is kept open over a certain area of the compression stroke. The drag forces exerted by the the gas pushed back through the held-open suction valve close the sealing element of the respective suction valve after overcoming a certain portion of the piston stroke, since said sealing element is loaded from the opposite side with a counteracting force adjusted according to the desired capacity reduction. The greater this counteracting force is, the later the respective suction valve in the compression stroke closes, resulting in the reduction of the capacity. Since there is a maximum drag force the suction valve no longer closes at all if the constant counteracting force is larger than said maximum closing force, the control range of this type of compressor control the maximum downturn of capacity must be limited to avoid interim idling of the compressor including all associated problems. For such reverse flow control embodiments are known, whose valve unloader is simply hydraulically or pneumatically prestressed for the suction valve to be held open, while control action can be exerted on the delivery rate by varying the corresponding prestressing pressure.

Furthermore, a reverse flow control for reciprocating compressors is for example known from EP 694 693 A1, where a hydraulic control cylinder, which can be periodically energized and relieved via a control element by a pressure medium synchronous to compression cycles, loads the valve unloader and thus the sealing element of the suction valve to be held open in stroke direction. This way the hydraulic lifting force is abruptly reduced at a certain stroke angle, thus initiating a safe and rapid closing of the suction valve. Similar delivery rate controls also with pneumatic actuation are known from EP 1 400 692 A1, which has the advantage that the actuation pressure can be directly maintained by the reciprocating compressor itself. Due to the relatively high compressibility of the compressed actuation gas, however, careful dimensioning of switched gas volumes and precise setting of actuation time is necessary.

Furthermore, reciprocating compressors with an electrically operated reverse flow control of the initially mentioned type have also been known for a very long time. For example from DE 1 251 121 A or from DE 849 739 B and similar patents, in part dating back as early as the 1930s, where a valve unloader engaging at the sealing element of the suction valve is moved via an electromagnet mounted at a valve cap, which is periodically excited by, for example, a collemutator, said commutator being synchronously rotated with the crank shaft of the compressor. Due to the—in part—very high reverse flow forces acting upon the sealing element of the suction valve large solenoids with a correspondingly large power consumption are required.

It is the task of the invention to develop a reciprocating compressor with electrically operated reverse flow control in such a way that the known arrangements of the type as described are improved in terms of reducing overall size and power consumption while still making it possible to provide high actuating forces with high motion dynamic—similar to the hydraulic reverse flow controls as described.

SUMMARY OF THE INVENTION

For a reciprocating compressor with capacity control of the type as initially described this task is solved in that the unloading device comprises as drive an electric motor, whose driven element consists of a guide surface—extending in a sloped manner in rotational direction—for low-friction rolling elements, which on the other hand interact with a counter-guide extending in a sloped manner at a non-rotating lifting element, the latter being connected to the valve unloader. This way, a relatively small-dimensioned electric motor can be used, whose rotational movement can be converted into the required hold-open force or motion acting on the valve unloader via the pairing of rolling elements extending in a sloped manner simply and with high actuating force as well as with high motion dynamics.

Particularly advantageous in this connection is a further embodiment of the invention, according to which the driven element of the electric motor in conjunction with the non-rotating lifting element and the rolling elements arranged in between form a ball screw drive. Such drives are known and in use in the most diverse contexts as nearly friction-free linear drives, and make it possible to exert relatively extensive control on the actuating force and dynamic by converting a number of revolutions of the electric motor—limited only by length and incline—into the resulting linear motion.

Another preferred embodiment of the invention comprises a simpler design, but a smaller diversity of possible variations, according to which the driven element of the electric motor in conjunction with the non-rotating lifting element and the rolling elements arranged in between form an inclined ramp drive permitting only a limited angular rotation. This allows only for a portion of the electric motor's revolution to be used to generate the linear motion, however with significantly lower design cost and also with the possibility of using cylinders, truncated cones or tons instead of balls as rolling elements, which may make sense with higher actuating forces.

In a further preferred embodiment of the invention the electric motor is developed as a brushless direct current motor comprising surface-mounted bandaged permanent magnets, which allows for a high dynamic with small construction. It is particularly preferred in this context to develop the electric motor with a hollow-shaft rotor, with the nut of the ball screw drive integrated, which minimizes the inertia of the rotor or of the rotary drive overall.

For the standard range of applications of such a reverse flow control according to the invention it turned out be particularly preferable for the hollow-shaft rotor to have a diameter ranging between 30 and 50 mm in a preferred additional embodiment, with the incline of the ball screw drive preferably ranging between 6 and 12 mm. This results in particularly well adapted circumstances for high actuating forces with a high dynamic of the lifting control simultaneously.

A further embodiment of the invention provides that the pairing of rolling elements at least in part consist of ceramics. The extremely high acceleration and brake momentums of the electric motor drive that occur under certain circumstances may lead to the inability of the rolling elements to follow the movement of their guide surface directly, which would result in a sliding on the guide surfaces and thus to wear. This problem can be resolved by using ceramic bearings or hybrid bearings comprising ceramic rolling elements with corresponding prestressing.

To accelerate and/or brake the actuation device's movement additional spring elements may be provided, allowing for the relief of stress on the electric motor drive with appropriate adjusting.

In a further preferred embodiment of the invention switching elements for dynamic braking and for temporary storage and reuse of the electric energy recovered therefrom may be provided in the electric drive of the electric motor. The energy may preferably be stored temporarily in capacitors, during the process of which the generated and stored current may be used either for the next actuation process at the same valve or at a different suction valve of the compressor. This way, the energy consumption to actuate the valve unloader is significantly reduced and, simultaneously, less lost heat is generated also within the system, thereby reducing the thermal stress and avoiding cooling problems in the actuation device.

A further preferred embodiment of the invention, according to which the current unloading force is adjusted to the progress of the reverse flow force, also serves to reduce unnecessary heating of the actuation device. While the suction valve is kept open, the level of the reverse flow force depends on the current piston speed, which for example is derived from a crankshaft angle sensor. The motor current for generating the necessary hold-open force may then be adjusted accordingly.

The invention is explained in detail below based on the embodiments schematically illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section through the area of a suction valve of a position compressor with delivery rate control according to the invention,

FIG. 2 shows an alternative embodiment of an electrically operated actuation device for a reciprocating compressor with delivery rate control according to the invention in a partial and schematic cross-section, and

FIG. 3 shows a detail of a further alternative embodiment of the drive of a actuation device for such a compressor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, a reciprocating compressor—not further shown—comprises a valve unloader 2 arranged at an automatically acting suction valve 1 of the compressor, said valve unloader holding open two ring-shaped sealing elements 4 of the suction valve 1 over a controllable portion of the compressor's cycle by means of an electrically operated actuation device 3. For this purpose, the actuation device 3 comprises as drive an electric motor 5, whose driven element 6 (developed here as a central shaft) comprises a guide surface 7 (here circumferential spiral groove) extending in an inclined manner in rotational direction for low-friction rolling elements 8, which on the other hand interact with a counter-guide 9 extending in an inclined manner at a non-rotating lifting element 10, the latter being connected to the valve unloader 2.

As soon as the electric motor 5 is supplied with current, its driven element 6 rotates, which, in conjunction with the non-rotating lifting element 10 and the rolling elements 8 arranged in between, forms a ball screw drive here, causing the lifting element 10, or its actuation plunger 11, in the illustration to move upward or downward. Thus, the valve unloader 2 is also moved upward or downward, and thus acts on the otherwise free mobility of the sealing elements 4. Contrary to the automatic actuation, which otherwise occurs solely via the pressure conditions upstream and downstream the suction valve 1, said sealing elements 4 can thus be kept open during a controllable portion of the compressor's compression stroke, allowing in a known manner for the delivery rate of a compressor—running at a constant rotational speed—to be regulated via so-called reverse flow control.

According to FIG. 2, the electric motor 5, preferably developed as a brushless direct current motor with surface-mounted bandaged permanent magnets 12, comprises as driven element 6 a hollow-shaft rotor 13, in which the nut 14 of the ball screw drive is integrated. The hollow-shaft rotor 13 preferably has a diameter ranging between 30 and 50 mm, with the incline of the ball screw drive formed by the nut 14, the rolling elements 8 and the non-rotating lifting element 10 preferably ranging between 6 and 12 mm. The hollow-shaft rotor 13 is mounted at the stator housing 15 on top of the bearing 16. The stator working together with the bandaged permanent magnet 12 is identified as 17.

According to FIG. 3, the driven element 6 of the electric motor 5, in conjunction with the non-rotating lifting element 10 and the rolling elements 8 arranged in between, forms an inclined ramp drive permitting only a limited angular rotation, which for example also allows the use of cylinder-shaped, truncated cone-shaped or ton-shaped rolling elements 8, with said elements being able to transmit greater axial forces.

The elements 6, 7, 8, 9, 10 of the rolling element pairings of the ball screw or inclined ramp drive may at least in part also be comprised of ceramics, which, in conjunction with corresponding prestressing of the drives, prevents sliding of the rolling elements 8 in case of highly dynamic actuations with the associated danger of damaging the rolling-body guidances.

To accelerate and/or brake the movement of the actuation device 2 additional spring elements 18 may also be provided, as indicated only schematically in FIG. 1.

As only indicated in FIG. 1, switching elements 20 for dynamic braking and for temporary storage and reuse of the electric energy recovered therefrom may be provided in the electric control device 19 of the electric motor 5, preventing unnecessary heating or the need for additional cooling of the actuation device. Furthermore, additional switching elements 21 for adapting the hold-open force of the actuation device 3 to the respective acting reverse flow force by controlling the supply of the electric motor 5 with current may also be provided in the control device 19, which also helps to reduce unnecessary loss of heat. 

1. A reciprocating compressor with capacity control comprising a valve unloader (2) arranged at at least one automatically acting suction valve (1) of the compressor, said valve unloader holding open at least the sealing element(s) (4) of one suction valve (1) over a portion of the compressor's cycle by means of an electrically operated actuation device (3), characterized in that wherein the actuation device (3) comprises as drive an electric motor (5), whose driven element (6) comprises a guide surface (7)—extending in a sloped manner in rotational direction—for low-friction rolling elements (8), which on the other hand interact with a counter-guide (9)—extending in a sloped manner—at a non-rotating lifting element (10), the latter being connected to the valve unloader (2).
 2. The compressor in accordance with claim 1, wherein the driven element (6) of the electric motor (5), in conjunction with the non-rotating lifting element (10) and the rolling elements (8) arranged in between, form a ball screw drive.
 3. The compressor in accordance with claim 1, wherein the driven element (6) of the electric motor (5), in conjunction with the non-rotating lifting element (10) and the rolling elements (8) arranged in between, form an inclined ramp drive permitting only limited angular rotation.
 4. The compressor in accordance with claim 1, wherein the electric motor (5) is developed as a brushless direct current motor comprising surface-mounted bandaged permanent magnets (12).
 5. The compressor in accordance with claim 2, wherein the electric motor (5) comprises a hollow-shaft rotor (13), in which the nut (14) of the ball screw drive is integrated.
 6. The compressor in accordance with claim 5, wherein the hollow-shaft rotor (13) comprises a diameter ranging between 30-50 mm, with the incline of the ball screw drive preferably ranging between 6 and 12 mm.
 7. The compressor in accordance with claim 1, wherein the elements (6-10) of the rolling element pairings consist at least in part of ceramics.
 8. The compressor in accordance with claim 1, wherein additional spring elements (18) are provided for accelerating and/or braking the movement of the actuation device (3).
 9. The compressor in accordance with claim 1, wherein switching elements (20) for dynamic braking and for temporary storage and reuse of the electric energy recovered therefrom may be provided in the electric control device (19) of the electric motor (5).
 10. The compressor in accordance with claim 1, wherein switching elements (21) for adapting the hold-open force of the actuation device to the respective acting reverse flow force are provided in the electric control device (19) of the electric motor (5) by controlling the supply of the electric motor (5) with current.
 11. A method for controlling the delivery rate of a reciprocating compressor in accordance with claim 1, wherein the braking energy to be applied for braking the drive of the actuation device (3) is generated by the dynamic braking of the electric motor (5) and the current generated in this process is temporarily stored and reused.
 12. The method in accordance with claim 11, wherein the hold-open force of the actuation device (3) is adjusted to the respective acting reverse flow force by controlling the supply of the electric motor (5) with current. 